1. Field of the Invention
The present invention relates generally to control of power supplies, and more specifically to methods and apparatus for converting from a widely varying input voltage, such as a rectified AC voltage or a filtered DC voltage, to an output current or voltage.
2. Background of the Invention
Cascaded converters have been demonstrated which efficiently convert widely varying input voltages, such as a rectified AC input voltage, or DC voltages such as those found in an automotive or truck battery voltage, to voltages or currents significantly larger or smaller than said inputs. Certain of these converters include a single switch cascaded converter such as a buck-boost-buck converter.
An efficient controller for these types of converters would divide out changes in frequency with input line; would allow sensing the negative voltages associated with such topologies; and would contend with the widely varying currents or voltage magnitudes and polarities being sensed to feedback output information to these topologies.
An example application for such converters would be the driving of high power LEDs. These LEDs require a fixed DC current and may be dimmed either by varying the current magnitude or producing a current pulse train which is better for LED lifetime.
Solid state illumination sources, LEDs, capable of replacing incandescent and fluorescent lamps have recently been demonstrated. These LEDs last longer and are more efficient than existing sources, however, the heat they produce is conducted rather than radiated. The result is extremely high operating temperature beyond the capability of standard power supplies containing electrolytic capacitors. Further, light fixtures typically do not have space for a power supply and a lamp, therefore to retrofit existing applications extremely small power supply form factors must be created.
It would therefore be desirable to incorporate the above controller into an integrated circuit which can operate from high input voltages, stay operational for a non-filter input such as a rectified AC signal or a phase controlled AC signal, and to provide open load, under voltage and other protections for the power supply. It would be desirable if the resulting supply was small enough to mount in retrofit form factors, is capable of high temperature operation for long periods of time, could regulate an output current, and could be mounted close to the illumination source.
It would be desirable in one embodiment to provide a power factor corrected power supply such that the power supply looks to the AC line like a resistive element.
It would further be desirable in another embodiment to reduce filter requirements by drawing constant power or near constant power through the converter so that input current varies inversely with instantaneous input voltage. To prevent over currents during the “cusps” or low voltages observed in an rectified AC waveform, it would be desirable to engage a filter only once reaching a voltage low enough to allow use of low ESR capacitors so that filtering components values and sizes may be minimized and so that components such as electrolytics with poor high temperature reliability may be eliminated from the circuit.
Therefore, it would be desirable to provide a method and apparatus for overcoming the problems discussed above.